Fix corrupt Mac App Store downloads

Apple's Mac App Store, which was introduced in OS X 10.6.6, has become Apple's central location for distributing new software as well as managing updates to the OS and purchased applications. Though in most situations the Mac App Store works fine to download and run a specified installer, there are times when errors occur and even persist regardless of how many times you retry the download.

MacFixIt reader Silverio recently wrote in with such a problem:

I've been experiencing problems to install updates for iMovie through Apple Store. The error message says that the product distribution file could not be verified. Is there an easy fix for this?

To protect application integrity, Apple includes various checksum routines, which simply treat any downloaded file as a lump of raw binary data and perform summations of the bits in this raw data according to a specified algorithm (MD5, SHA1, etc.), after which you get a large number that represents a unique characterization of the file's integrity. Developers and Apple create these signatures by running the checksums before the applications are distributed and include them with the program.

When you download and open the checksummed program on your computer, the same summation algorithm is run on the file and its results are checked against the stored signature. If they match, you have an installer that is assumed to be intact, but if not, then an error occurred that may have compromised the integrity of the installer and may result in an unstable or harmful installation.

Checksum errors can happen for various reasons, including malicious tampering with the file (usually rare), slight alterations to the file and its metadata (including the initially stored checksum signature) as it is stored both on the server and on your computer, and more commonly because of interruptions during data transfer. Regardless of the reason, if checksum errors occur and the file cannot be verified, then it is best to re-download the file to ensure that you get a fully intact copy.

The App Store will save downloaded application installers to a hidden cache; if an integrity error occurs, then the store should remove this downloaded file and start over. However, problems may prevent this from happening and result in the App Store attempting to run the cached faulty installer every time instead of re-downloading a new version to try.

If this type of error happens, the best approach is to dig into your system and remove the cache for the App Store, which should clear out the faulty installers and make the store re-download them. To do this, open the Terminal utility (in the /Applications/Utilities/ folder) and run the following command (copy and paste it):

open $TMPDIR../C/

This will open a directory that holds various caches for applications and system services, so locate the one for the App Store, which should be a folder called "com.apple.appstore." Once found, you can either open the folder and try removing the installation package for the program itself, or simply remove the entire folder (it will be recreated when the App Store is next run).

After removing the cache or the installer within it, try running the App Store again and downloading your desired program.

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